In co-pending application Ser. No. 07/200,204 filed May 31, 1988, apparatus and a method for continuously cooking pulp to enhance the quality of the pulp is disclosed. The apparatus and method therein provides for reduced mechanical handling of, and mechanical action on, the wood chips (or other comminuted cellulosic fibrous material that is utilized), and energy efficiency problems, quality drawbacks, and size limitations typically found in conventional cooking systems are minimized. According to the present invention, an alternative arrangement is provided which also has minimal mechanical action on the chips (especially where the temperature and pressure are high), and which should provide for reduced cost of operation, horsepower requirements, and pulp degradation. Also packings and seals that are common in conventional systems are unnecessary.
The internal impregnation vessel that is utilized in a number of the embodiments of said co-pending application Ser. No. 07/200,204 was under full digester pressure, and therefore the shell, flanges, nozzles, pipes, etc. had to be dimensioned accordingly. According to the present invention, however the actual internal impregnation vessel is also at digester pressure, however it comprises a non-pressurized tube since the pressure is the same on both sides of the vessel wall. Thus it is a mere standpipe, rather than a pressurized vessel (as in Ser. No. 07/200,204).
The impregnation vessel according to the invention passes the liquid slurry of fibrous cellulosic material upwardly to a solids/liquid separator located adjacent the top of the digester. Liquid that is separated is passed downwardly in a conduit welded or otherwise attached to the impregnation vessel, to ultimately pass out the bottom of the vessel. In this way, the minimum mechanical action on the chips achieved with said co-pending application apparatus and method is also substantially possible, while other advantages accrue.
According to one aspect of the present invention an apparatus for the continuous cooking of cellulosic fibrous material to produce paper pulp is provided. The apparatus comprises the following elements: (a) A generally vertically disposed impregnation vessel having a bottom, and a top in open communication with the digester pressure. (b) Means for feeding a liquid slurry of cellulosic fibrous material to the bottom of the impregnation vessel. (c) A mechanical separating means disposed at the top of the impregnation vessel for separating cellulosic fibrous material from liquid, so as to decrease the liquid to material ratio of the slurry. (d) A generally vertically disposed digesting vessel having a top and a bottom, and an inlet at the top thereof. (e) The top of the impregnation vessel located adjacent the top of the digesting vessel, and the impregnation vessel disposed within the digesting vessel, generally concentric therewith. (f) A liquid return conduit open to the digesting vessel pressure, and disposed within the digesting vessel adjacent the impregnation vessel, for returning liquid separated by means (c) to exit the bottom of the digesting vessel. (g) Extraction screen means disposed in the digesting vessel for withdrawing liquid therefrom. And, (h) pulp discharge means disposed adjacent the bottom of the digesting vessel.
According to another aspect of the present invention, a method of continuously digesting comminuted cellulosic fibrous material to produce paper pulp is provided. The method comprises the steps of substantially sequentially and continuously: (a) Feeding a liquid slurry of comminuted cellulosic fibrous material to the bottom of the impregnation vessel. (b) Flowing the material (at a temperature of about 100.degree.-120.degree. C.) upwardly in the impregnation vessel from the bottom to the top thereof, the impregnation vessel being open at the top to the digesting vessel pressure. (c) Separating some of the liquid from the slurry at the top of the impregnation vessel. (d) Moving the material from the bottom of the impregnation vessel to the top of the digesting vessel with a minimum of mechanical action on the material. (e) Withdrawing liquid separated in step (c) from the digesting vessel by passing it downwardly through the digesting vessel open to digesting vessel pressure out the bottom of the digesting vessel. (f) Digesting the material (at a temperature of about 160.degree.-170.degree. C.) in the digesting vessel, utilizing digesting liquid. And, (g) withdrawing pulp from the bottom of the digesting vessel.
It is the primary object of the present invention to provide for the enhanced continuous cooking of cellulosic fibrous material. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.